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Good morning. Welcome to Mersana's first-quarter 2020 conference call. We issued a press release earlier this morning reviewing our first-quarter 2020 results and business updates, which will be covered on this call. A replay of today's call will be available on the Investors & Media section of our website.

After our prepared remarks, we will open the call for Q&A. Before we begin, I'd like to mention that our call will contain forward-looking statements within the meaning of federal securities laws. These are not statements of historical facts and are based on management's beliefs and assumptions and on the information currently available. They are subject to risks and uncertainties that could cause the actual results and the implementation of the company's plans to vary materially, including the risks that are early encouraging preclinical results for XMT-1536 and XMT-1592, are not necessarily predictive of the results of our ongoing or future discovery programs or clinical studies, as the development and identification of our company's product candidates and new platforms will take longer and/or cost more than planned, and that our clinical trials will not be completed on schedule, if at all. These risks are discussed in the company's SEC filings, including, without limitation, the company's annual report on Form 10-K filed on February 28, 2020, and subsequent filings.

In addition, while we expect that the COVID-19 pandemic might adversely affect the company's preclinical and clinical development efforts, business operations and financial results, the extent of the impact on the company's operations and the value of and market for the company's common stock will depend on the future developments that are highly uncertain and cannot be predicted with confidence at this time, such as the ultimate duration of the pandemic, travel restrictions, quarantines, physical distancing and business closure requirements in the U.S. and in other countries and the effectiveness of actions taken globally to contain and treat the disease. Except as required by law, the company assumes no obligation to update these forward-looking statements publicly, even if new information becomes available in the future. With that, I'll turn the call over to Anna Protopapas, Mersana's president and chief executive officer.

Since the beginning of 2020, we made great strides toward advancing our programs and executing against our key milestones. Although, the COVID-19 pandemic has impacted us in the world in unprecedented ways, we quickly adopted the way we do our work to ensure the safety of our employees and our patients, while continuing to advance our key programs with a focus on delivering on our corporate goals and milestones. I will provide a high-level overview of what we have achieved this quarter, and then Dirk, Tim and Brian will provide more details. Before I begin, today is world ovarian cancer day, and I'd like to take a moment to recognize the women living with ovarian cancer, their families and the many patient advocacy groups around the world promoting awareness for this devastating disease.

There remains a significant unmet medical need and limited treatment options for these women. Today, we at Mersana reiterate a supported commitment to developing life-changing ADC therapeutics for those affected by ovarian cancer. With that, I would like to turn to the exciting data we recently presented from the Phase 1 dose escalation study of 1536 in heavily pretreated patients with ovarian cancer and lung adenocarcinoma. These data demonstrate that XMT-1536 has a favorable safety and tolerability profile without the severe neutropenia, neuropathy or ocular toxicity seen with other ADCs.

We saw confirmed responses in durable stable disease in heavily pretreated patients who have exhausted all other options and showed an emerging biomarker response relationship. We also announced that we had reached the maximum tolerated dose at 43 milligrams per meter squared and are no longer enrolling patients to the dose-escalation portion of the study. Second, we remain on track to disclose early data from the ongoing expansion study of XMT-1536 and are very pleased to report that our abstract has been accepted for a poster session at the ASCO 2020 virtual annual meeting to be held between May 29 and May 31. In advance of the virtual ASCO poster session, we plan to hold a conference call and webcast to discuss this data on May 27 at eight a.m.

Eastern Standard time. We will be joined on the call by Dr. Debbie Richardson, associate professor gynecological oncology at the Stevenson Cancer Center at the University of Glaucoma Health Care Center and the Sarah Cannon Research Institute. As we have indicated in the past, this interim disclosure of the expansion cohorts will be heavily skewed toward ovarian cancer.

Dirk will provide more details on the scope of our disclosure, but we believe we have accumulated meaningful patient experience to continue to support favorable safety and tolerability, a promising activity profile and to further establish a biomarker response relationship. A third important milestone for us is initiating dosing of patients in the XMT-1592 Phase 1 dose-escalation study in the first half of this year. We believe we're on track to achieve this goal, an important first step in better understanding the potential clinical differentiation of XMT-1592. A fourth set of goals is associated with our earlier-stage pipeline.

We remain on track to reach our ADC development candidate milestones this year. We have initiated IND-enabling studies with B7-H4 and are on track to disclose data on this candidate in the second half of the year. In addition, we're continuing to advance our immunosynthen platform and remain on track to select our first immunosynthen candidate in the second half of the year. Finally, we added $65 million in gross proceeds through our ATM with participation based on interest received from Avoro, Bain Capital, Consonance Capital and our chairman, Dave Mott.

This strengthens our balance sheet and will be used to support our operations as we continue to advance our pipeline of innovative ADC candidates. With that overview, I would like to pass it on to the team, who can give you more details on the progress we have made across all aspects of our business. Let me turn the call over to Dirk, first, to discuss 1536.

Dirk Huebner -- Chief Medical Officer

Thanks, Anna, and good morning, everyone. As Anna mentioned, we were very pleased to present positive data from the Phase 1 dose-escalation study of XMT-1536 and heavily pretreated patients with ovarian cancer and NSCLC adenocarcinoma, two areas of significant unmet medical needs. The cutoff date for this evaluation was February 3 and included 59 patients. 37 were ovarian cancer patients, 11 had NSCLC and the remainder were patients with more rare tumors potentially expressing NaPi2b.

I will now highlight the key findings of the study presented by our share of the safety review committee, Dr. Debra Richardson. This data showed that XMT-1536 is well tolerated without the severe toxicities of other ADC platforms, such as neutropenia, peripheral neuropathy or ocular toxicity. The most common treatment-related adverse events were grade one and two nausea, fatigue, headache and the most frequent grade three treatment-related AE was transient AST elevation.

There were no dose-limiting toxicities observed in the 43-milligram per square meter cohort. As a reminder, these were patients with a median of five prior lines of therapy, and in most cases, had run out of other treatment options. Longitudinal studies have shown that as ovarian cancer patients progress, the potential for response decreases rapidly with lines of therapy and that the expected response rate is close to zero for these ovarian cancer patients. We saw further confirmed partial responses in ovarian cancer and our first confirmed partial response in an NSCLC adenocarcinoma patients.

The fact that we saw confirmed partial responses in durable stable disease in this patient population is very encouraging and supports further development of XMT-1536 in the ongoing dose-expansion study. In addition, a favorable biomarker response trend was observed. For the subset of evaluable patients treated at 30-milligram per square meter and above who had higher NaPi2b expression, five of 15 or 33%, of patients achieved partial responses and six out of 15 or 40% of patients achieved stable diseases for a disease control rate of 11 out of 15 or 73%. In contrast, there were no responders in the low NaPi2b-expressing group.

This is important because it indicates the potential for a patient-selection strategy which can identify patients most likely to benefit from XMT-1536. Before I move on to XMT-1536 dose-expansion study, I'd like to remind that the first two patients in the 52-milligram per square meter dose-escalation cohort experienced dose reductions, which allows us now to declare the 43-milligram per square meter dose as the MTD for XMT-1536. The dose-escalation portion of the study is now closed for new enrollment. As Anna mentioned, our XMT-1536 expansion abstract was accepted by ASCO and will be presented on a conference call and in a poster session at the ASCO 2020 virtual meeting being held May 29th to 31st.

The data will include safety, tolerability and efficacy for patients treated with 36-milligram per square meter and 43-milligram per square meter. As a reminder, the expansion study, which was initiated at 36-milligram per square meter, in August 2019, was later amended to enroll patients at 43-milligram per square meter and continue to enroll patients at this dose level. With the cutoff date of May 1, 2020, the presentation will include 20 RECIST-evaluable ovarian cancer patients and four RECIST-evaluable NSCLC patients. Biomarker expression data will be available for the majority of evaluable patients.

Additional patients are enrolled in the study but have not yet reached the RECIST evaluation time point. Given the limited duration of follow-up today, we do not expect to have mature data on duration of response. We expect to provide more information on durability of response when we report more mature data. Remember that the ovarian cancer and lung adenocarcinoma patients being treated in the dose-expansion study are still heavily pretreated and platinum-resistant but more homogeneous with one to three prior lines of therapy in ovarian cancer and some patients having four prior lines of therapy, regardless of platinum status.

In NSCLC adenocarcinoma patients have failed chemotherapy, PD-1, either alone or in combination, and those harboring an oncogenic driver mutation pair-targeted therapy. Our goal is to enroll 40 to 45 patients in both ovarian cancer and NSCLC adenocarcinoma cohorts. As we have said in the past, we believe the expansion cohorts will provide proof-of-concept for XMT-1536 and will allow us to shut the path to approval, particularly in ovarian cancer where there is precedent for a single-arm registration trial. I will now turn the call over to Tim Lowinger to discuss our research and development work.

Tim Lowinger -- Chief Science and Technology Officer

Thanks, Dirk, and good morning, everyone. I will start with our first-in-class B7-H4 ADC development candidate. We're excited about this ADC target and believe it is well suited for the unique characteristics of our DolaLock payload and platforms. We have compelling preclinical efficacy and nonhuman primate tolerability data with both Dolaflexin and Dolasynthen ADCs targeting B7-H4.

Initiation of IND-enabling studies is ongoing, and we recently initiated cell-line development. We remain on track to disclose our development candidate and the supporting data in the second half of this year. We've also continued to make important progress in our Immunosynthen platform design and the selection of our first STING agonist ADC clinical candidate. As a reminder, the goal of the Immunosynthen platform is to take ADCs beyond the traditional cytotoxic payloads to immunomodulatory payloads, and preclinical studies show the potential to be safely and efficiently delivered to the tumor and stimulate an antitumor innate immune response.

We have a robust set of efficacy and tolerability preclinical data across multiple targets and remain on track to finalize the Immunosynthen platform design and select our first STING agonist ADC development candidate in the second half of 2020. We are excited for the potential of this platform and believe that activating the innate immune system to fight cancer in a targeted manner is a potential game changer. Before I turn the call over to Brian, I would like to note that, last quarter, we announced that we planned to present preclinical data on ADCs created with the Dolasynthen and Immunosynthen platforms at the American Association for Cancer Research annual meeting. Since that time, AACR has changed the meeting to a virtual format, and we will now be presenting this data as part of a virtual poster session scheduled for June 22, 2020.

These data will include preclinical data from our work on XMT-1592, a Dolasynthen ADC targeting NaPi2b and our novel Immunosynthen STING agonist ADC platform. The abstracts outlining these data will be made available by AACR on May 15. We look forward to being able to share this exciting work. And with that, I will turn the call over to Brian DeSchuytner for an overview of our financial results.

Thank you, Tim. Good morning, everyone, and thank you for joining us today. Today, I'll review some of the key financial highlights from our first-quarter 2020 results, and I'll start with our cash position. We ended the first quarter of 2020 with approximately $78.4 million in cash, cash equivalents and marketable securities, compared to approximately $100 million at the end of 2019.

Net cash used in operating activities in the first quarter was $21.2 million, compared to $24.7 million in the same period in 2019. Net cash used in operating activities increased by approximately $9 million when compared to the fourth quarter of 2019, mainly due to the timing of compensation payments, as well as a reduction on the accounts payable balance. As a reminder, in 2018, we set up at the market or ATM financing facility in the amount of $75 million. On April 7, we announced that we had raised gross proceeds of approximately $65 million through this facility with participation based on interest received from Avoro Capital Advisors; Bain Capital Life Sciences; Consonance Capital Investors; and David Mott, Mersana's chairman of the board.

Through this transaction, we sold approximately 8.9 million shares of the company's common stock at a purchase price of $5.59 and approximately two million shares at a closing price of $7.74. In each case, the market price at the time of the sale. In addition, we have the option to draw additional funds of up to $15 million through the existing debt financing agreement with Silicon Valley Bank. Today, we are providing updated cash runway guidance following the ATM transaction.

We expect that our current cash, cash equivalents and marketable securities, including the proceeds from the ATM facility, will enable us to fund our operating plan into early 2022. And now some of the key highlights from our first-quarter 2020 financial results. Collaboration revenue in the first quarter of 2020 was immaterial when compared to the $41 million for the same period in 2019. This decrease in collaboration revenue was primarily a result of the recognition of $40 million in deferred revenue in the first quarter of 2019 associated with the discontinuation of the partnership with Takeda.

Research and development expenses for the first quarter of 2020 were approximately $12.2 million, compared to $15.1 million for the same period in 2019. The variance was primarily due to an upfront payment for a technology license fee in 2019 and the timing of research efforts, the decrease in expenditures in support of partner programs and a decrease in manufacturing costs for 1536 and 1522, all offset by an increase in XMT-1536 clinical expenses, as well as the advancement of the companion diagnostics development efforts for the NaPi2b biomarker. General and administrative expenses for the first quarter of 2020 were approximately $4.9 million, compared to $4.4 million in the same period in 2019. And net loss for the first quarter of 2020 was $16.9 million or $0.35 per share, compared to a net income of $21.9 million or $0.72 per share for the same period in 2019, the difference was attributable over -- year over year was attributable to that $40 million in deferred revenue recognized in the first quarter of 2019.

Weighted average common shares outstanding for the quarters ended 31st of March 2020 and the 31st of March 2019 were approximately $48 million and $30 million, respectively. Common shares outstanding as of May 5, 2020, were approximately $59 million. This count includes the shares issued in connection with the April ATM transaction, as well as the exercise of approximately $2.6 million prefunded warrants by BVF, pursuant to the common stock exchange agreement signed in November 2019. I will now turn the call back to Anna.

Anna Protopapas -- President, Chief Executive Officer, and Director

Thank you, Brian. The COVID-19 pandemic has brought challenges for all of us. I'm proud of the Mersana team's commitment to science and patients and the team's ability to continue to execute on our goals in the midst of such a disruption. I'd like to take a moment to discuss the impact of the pandemic and mitigation strategies we have put in place to reduce risk and ensure business continuity as best we can.

First, and in line with the guidance from the U.S. Center of Disease Control and Prevention in the state of Massachusetts, we have implemented work-from-home measures for all non-lab employees and have suspended all business travel. We have also implemented processes to ensure we continue to advance our ADC development candidates while working to ensure the safety of our team, while most of our lab employees also continue to work from home. We have prioritized lab activities and have implemented stagged schedules in the interest of safety and efficiency.

We will continue to monitor the situation and continue to follow federal, state and local guidelines and directives. From a clinical study perspective, the COVID-19 pandemic is a burden on all sites and hospitals in the U.S. and other countries, but cancer patients will always need treatment. We have taken steps where possible for remote monitoring and remote testing, consistent with FDA guidance, and remain focused on ensuring the patients we are benefiting can continue their treatment.

We have over 20 sites up and running with geographic diversity across the U.S. These sites now have experience with XMT-1536 and are excited about its potential benefit to patients and are continuing to enroll patients in the trial. Though screening of new patients has slowed down to some extent. Additionally, we had plans to bring new sites up, and some of those activities are ongoing, but certain sites have put initiation of new trials on temporary hold.

Lastly, on the manufacturing front, we have sufficient inventory of XMT-1536 and XMT-1592 to support ongoing and planned clinical studies and sufficient inventory of advanced intermediates stockpiled in the U.S. to support greater than two years of manufacturing of drug substance and drug product. Overall, we feel good about where we are on this front. Finally, before I turn the call over for questions, I would like to provide an update on our expected milestones for the rest of the year.

I'll start with 1536. As I mentioned, we remain on track to provide an interim look at the expansion study data on a conference call and webcast and in a virtual poster session at ASCO in late May. We had previously guided to being able to provide a more mature look at this data in the second half of the year. Our objective is to continue to execute toward that goal.

If the COVID-19 disruption impacts our ability to deliver on that goal, we will provide guidance as appropriate. To reiterate, non-small cell lung cancer adenocarcinoma enrollment continues to lag behind ovarian, and this is likely to continue to be true in the context of the COVID-19 pandemic. For XMT-1592, we remain on track and look forward to dosing patients in the second quarter of 2020. As for our pipeline development candidates, and as Tim said, we believe we're still on track to disclose our B7-H4 and STING ADC development candidates and the supporting data in the second half of this year.

Overall, we are in a good place from a business continuity standpoint, and I thank our employees for this and admire their continued dedication and drive as we all work together to get through these uncertain times. We remain committed to achieving the milestones we laid out at the beginning of the year and reaching our overarching goal of significantly improving outcomes for people living with cancer. With that, I will turn the call over to the operator for Q&A.

Questions & Answers:

Operator

[Operator instructions] Our first question comes from the line of Jonathan Chang from SVB Leerink. Your line is now open.

Jonathan Chang -- SVB Leerink -- Analyst

Good morning, and congrats on the progress, and thanks for taking the questions. First question, based on other data we've seen in the platinum-resistant ovarian cancer space, one could make the argument that in earlier-line patient population, activity should improve. Would it be fair to have that expectation of your dose-expansion data set compared to your dose-escalation data set? Why or why not?

Anna Protopapas -- President, Chief Executive Officer, and Director

So thank you for the question, Jonathan. As we've said before, the patients we're treating in the expansion cohort are still heavily pretreated patients, platinum-resistant patients with three, potentially four lines of therapy. And as you know, the standard of care there is single-agent chemotherapy pegylated doxorubicin, topotecan, and the response rate there is pretty well established. The response rate for single-agent chemotherapy is really no better than a 12% response rate in PFS.

As you recall, from our dose-escalation data, we've seen confirmed responses in prolonged stable disease in these very heavily pretreated patients, significantly later-stage patients than what the expansion cohort population represents. So we've already achieved a significantly superior response rate in later-stage patients. And I think that establishes a level of activity that supports moving forward into a registration -- a custom market registration path. So could it get better with the earlier? Of course, it could.

But I don't necessarily think it has to have a meaningful drug that can really benefit patients and to have a path forward cost-to-market registration strategy.

Jonathan Chang -- SVB Leerink -- Analyst

Got it. And just second question, you indicate that the majority of the evaluable patients from the expansion will have biomarker data. I'm curious, has your thinking around the prevalence of NaPi2b expression and potential addressable population change with your experience in both the dose escalation and expansion?

Anna Protopapas -- President, Chief Executive Officer, and Director

I'm sorry, Jonathan, can you repeat the question?

Jonathan Chang -- SVB Leerink -- Analyst

Sure. You indicated that the majority of the valuable patients from the expansion will have biomarker data. I'm curious if you're thinking around the prevalence of NaPi2b expression and the potential addressable population has changed with the experience you guys have had in the dose-escalation and expansion portions?

Anna Protopapas -- President, Chief Executive Officer, and Director

So as we've said, we do not -- we haven't really established the cutoff, although we are encouraged with the biomarker response data we are seeing. What we'd like to do is get the totality of the data from all the patients in the expansion cohort and really use that to establish the cutoff. That being said, we continue to be encouraged that we are seeing a good correlation, and we continue to be encouraged that our understanding of the range of expression of NaPi2b in ovarian cancer patients remains consistent from the clinical experience as we did in all the work leading up to entering to the clinic, all the work we did with tissue samples. And we feel we understand the broad expression.

Jonathan Chang -- SVB Leerink -- Analyst

Thank you.

Operator

Thank you. Our next question comes from the line of Debjit Chattopadhyay from H.C. Wainwright. Your line is now open.

Earl DeSouza -- H.C. Wainwright and Company -- Analyst

Hi. Good morning, everybody. Happy Friday. Congrats on all the progress.

This is Earl DeSouza in for Debjit. Just a few questions from us. So of the majority of the 20 patients that you will be presenting, would that be from the 36-mg cohort? And I know you talked about it earlier during your remarks. But what is the length of follow-up with these patients? And a follow-up question would be -- I guess my second question would be what kind of durability of disease control would convince you to head toward a single-arm registration path?

Anna Protopapas -- President, Chief Executive Officer, and Director

So thank you for the question, Debjit. The durability really is -- it really would depend on -- as you can appreciate, we have patients that have been on study for just a few weeks, and we have patients that have been on study for six months or more. So as we have said in the past, the data -- the mature data will be disclosed in the second half of the year. It's a little premature to really determine the duration and -- of treatment and given that we have such variation in time of our patients being on the trial.

As for your question about the mixture of 36 and 43, I'd like to refer to my colleagues, maybe Dirk that answer.

Dirk Huebner -- Chief Medical Officer

Yes, this is Dirk. I welcome that question. As we've said on the call, the 36-milligram per square meter dose cohort was started in August last year. And from that perspective, we have certainly patients in that cohort.

We moved -- or started moving over to the 43 milligram in December. So we have patients in both the 36 and the 43 that are available for assessment at the presentation. It's split somewhat in half, if you like. But of course, because the 36-milligram patients are on study longer.

We want to have a little bit more duration on those patients compared to the 43.

Operator

Thank you. Our next question comes from the line of Boris Peaker from Cowen. Your line is now open.

Boris Peaker -- Cowen and Company -- Analyst

Good morning, and I'd like to add my congratulations on -- to the progress. My first question is, just curious, as you enroll more patients and obviously in these dose expansion arms, at which point do you anticipate to be in a position with kind of adequate data in hand to discuss the regulatory path forward in ovarian cancer with the FDA?

Anna Protopapas -- President, Chief Executive Officer, and Director

Great question, Boris, and one we are discussing internally. We had originally said and planned that we would recruit 40 to 45 patients. We thought that was a reasonable patient experience to make well-informed decisions and have a really meaningful discussion with the FDA. Obviously, as the data evolves, we're continuing to look at that and -- but that was the plan, and we remain on that plan at this point.

Boris Peaker -- Cowen and Company -- Analyst

Great. And just my follow-up question, just logistically, on the 1536 study, I'm curious, are these patients or the majority of these patients being treated in the hospital or the outpatient setting? And how frequently do they need to show up for the facility? And how long do they stay at every treatment?

Anna Protopapas -- President, Chief Executive Officer, and Director

Dirk, I think you might be able to better give an answer to that question.

And then what's usually happens thereafter, some collection of blood samples for different things, for lab values, pharmacokinetic collection, etc. So that happens on the first day. They come back on the second and third, but they don't stay in the hospital, and then we follow up on a weekly schedule with those patients.

Boris Peaker -- Cowen and Company -- Analyst

And just how long do they typically stay for each visit, like with the infusion and all the assessment? I'm just curious.

Dirk Huebner -- Chief Medical Officer

Yes. The first day is a little longer. That could be up to eight hours. The follow-up days would be shorter.

Boris Peaker -- Cowen and Company -- Analyst

Great. Thank you for taking my question.

You're welcome. Thank you.

Operator

Our next question comes from the line of Tom Shrader from BTIG. Your line is now open.

Tom Shrader -- BTIG -- Analyst

Good morning. Congratulations. I apologize If this is obvious to everyone else. But in May 20, is that 20 -- are those all new patients.

I remember there had been some dose experts or -- some of the doses had been expanded the last time we saw data. So is this all new patients?

Anna Protopapas -- President, Chief Executive Officer, and Director

The 20 patients that we are evaluable on the study. The plenty ovarian patients are part of the expansion cohort, and they're all different patients than the ones we disclosed in the dose escalation.

Tom Shrader -- BTIG -- Analyst

Great. And then if I could, a good nerdy question for Tim. On the Immunosynthen program, so much of the STING work has been dictated by direct tumor injection. Do you expect to go after the same tumors that has been done before? Are there hints in the literature where STING might be exciting but you couldn't get to with nonsystemic routes? Just your thoughts on whether you think there's low-hanging fruit in terms of tumor type for that kind of approach?

Tim Lowinger -- Chief Science and Technology Officer

Sure, Tom. I love the nerdy questions. I think we -- we're very pleased with the progress we're seeing in our Immunosynthen STING agonist ADC platform, and we look forward to disclosing the first candidate in the second half of this year. I think what I can say is that we have data, now, across multiple targets so it's not just a single target, and we think we have a very good understanding of what targets are appropriate and also which ones may not be.

And so I think -- and Tom I can't really say it today, but stay tuned in the second half, we'll disclose the data.

Tom Shrader -- BTIG -- Analyst

No. That's helpful. Thank you.

Operator

[Operator instructions] Our next question comes from the line of Jessica Fye from JP Morgan. Your line is now open.

Jessica Fye -- J.P. Morgan -- Analyst

Hey, guys. Good morning. Thanks so much for taking my questions. Can you please set the stage for the ASCO abstract and help us think about the difference in the amount of information we'll get in the abstract relative to the full presentation? How many evaluable patients will be in the abstract? And will we get safety there?

Anna Protopapas -- President, Chief Executive Officer, and Director

Great question, Jess. Thanks for asking it. If you can appreciate, the abstract was submitted in late January, early February. So I would guide you to look at it more as a placeholder rather than anything else.

The data really to look at is our disclosure on May 27, where we'll have the chance to really walk everyone through the full data set.

Jessica Fye -- J.P. Morgan -- Analyst

OK, great. And maybe second question, sort of building on an earlier one. In the press release, you talked about the ASCO data reporting on the relationship between response and biomarker expression. So when investors look at the response rate for the 20 ovarian patients at ASCO, should we look at ORR across the overall group or should we be oriented to a subset of higher expressors?

Anna Protopapas -- President, Chief Executive Officer, and Director

We will present both. And as we've said, we are -- we haven't yet definitively defined the cutoff, but we are, as we get more data, looking at that relationship. And as we did in the dose escalation, we'll be able to share both the total population, overall response rate as well as the response rate relation -- the response biomarker relationship.

Jessica Fye -- J.P. Morgan -- Analyst

Great. Thank you.

Operator

Our next question comes from the line of Mike Ulz from Baird. Your line is now open.

Mike Ulz -- Baird -- Analyst

Hi, guys. Thanks for taking the question. Congratulations on all the progress. I just had a question on XMT-1536.

You mentioned enrollment in non-small cell lung is sort of lagging behind ovarian. Can you just remind us, is that due to the timing of enrollment or there are other factors that might be driving that?

Anna Protopapas -- President, Chief Executive Officer, and Director

Look, I think we've always said that enrollment in lung cancer was not moving as fast as in ovarian. We started off with a lot more ovarian cancer patients in the dose escalation, primarily because we focus on sites that had -- that were primarily ovarian cancer recruiting sites. Often with success, we get more success as we saw more responses in ovarian, it accelerated recruitment in ovarian. We have been increasing our experience with lung, but it's lagging behind.

Other factors beyond just the board -- the relative POU data we have between ovarian and lung is that we are lung more -- there are more competitive trials in lungs. And we have a number of new sites that were planned to be up and running that we're focused on lung to date and have been partially delayed. We are still working toward bringing that recruitment up and believe we're on track to recruit the 40 patients we want in the expansion cohort this year. But we're continuing to monitor the situation.

And if that changes, we will let you know.

Mike Ulz -- Baird -- Analyst

Got it. That's helpful. And then maybe just another question on XMT-1592. So you're still on track to start your Phase 1 study this quarter.

So maybe if you could just talk about the study design there and maybe your dosing strategy as well?

Anna Protopapas -- President, Chief Executive Officer, and Director

Yes. We will be recruiting -- our objective in the dose escalation is move through dose escalation as quickly as possible. We will be recruiting both lung and ovarian cancer patients because that's the way we can move through dose escalation as quickly as possible. And the first few doses are single-patient dose cohort, and as we initiate the trial and start dosing patients, we'll be able to share more information with you about that trial.

Mike Ulz -- Baird -- Analyst

Thank you.

Operator

Thank you. Our next question comes from the line of David Nierengarten from Wedbush Securities. Your line is now open.

David Nierengarten -- Wedbush Securities -- Analyst

Hi. I just have a quick question. Will there be, and I understand it's difficult, a -- any opportunity to see longitudinal biomarker data over time? If you have archival samples and comparing them to fresh samples for the ovarian cancer -- or any of the patients? Just a question on that, the ASCO presentation.

Anna Protopapas -- President, Chief Executive Officer, and Director

Yes. It will not be part of the ASCO presentation. And at this point, it's not part of the protocol to get paired biopsies from patients we're treating. That's something we might embark on in the future, but it's not part of the current protocols.

David Nierengarten -- Wedbush Securities -- Analyst

OK. Understood. Thank you.

Operator

Thank you. At this time, I am showing no further questions. I would like to turn the call back over to Anna Protopapas, president and CEO, for closing remarks.

Anna Protopapas -- President, Chief Executive Officer, and Director

I want to thank everyone for participating on the call. We are very excited to have the opportunity to share with you the expansion corporate data in a few weeks and look forward to talking to you again at that time.